The invention relates to a method for operating a working machine.
The invention is applicable on working machines within the field of industrial construction machines, in particular wheel loaders. Thus, the invention will be described with respect to a wheel loader. However, the invention is by no means limited to a particular working machine. On the contrary, the invention may be used in a plurality of heavy working machines, e.g. articulated haulers, trucks, bulldozers and excavators.
Wheel loaders are generally provided with an internal combustion engine, a transmission line, a gearbox, driving wheels and a working hydraulic system.
The combustion engine provides power to the different functions of the wheel loader. In particular, the combustion engine provides power to the transmission line and to the working hydraulic system of the wheel loader.
The transmission line transfers torque from the combustion engine to the gearbox, which in turn provides torque to the driving wheels of the loader. In particular, the gearbox provides different gear ratios for varying the speed of the driving wheels and for changing between forward and backward driving direction of the wheels.
The working hydraulic system is used for lifting operations and/or for steering the wheel loader. For this purpose there are at least one hydraulic working cylinder arranged in the wheel loader for lifting and lowering a lifting arm unit, on which a bucket or other type of attachment or working tool is mounted for example forks. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front and rear body part of the wheel loader.
To protect the combustion engine of a wheel loader from sudden rapid changes in the working conditions of the gearbox and the driving wheels it is common to provide the transmission line with a hydrodynamic torque converter or similar arranged between the combustion engine and the gearbox. The hydrodynamic torque converter provides an elasticity that enables a very quick adaptation of the output torque to the changes in the working conditions of the gearbox and the driving wheels. In addition, a torque converter provides an increased torque during particularly heavy working operations, e.g. during acceleration of the wheel loader.
For example, if a wheel loader without the elasticity of a torque converter or similar is driven into an obstacle so that the vehicle stops this will also stop the combustion engine, since the engine in such designs is rigidly and unyieldingly connected to the rotation of the driving wheels. However, this will not happen if a torque converter or similar is arranged between the engine and the driving wheels or more preferably between the engine and the gear box. On the contrary, if the driving wheels of the wheel loader stops this causes the output side (the turbine side) of the torque converter (the turbine side) to stop whereas the input side (the pump side) continues to rotate together with the engine. The engine will experience a larger internal resistance from the torque converter but it will not come to a standstill.
However, the elasticity of a hydrodynamic torque converter or similar is not present between the working hydraulic system and the combustion engine. On the contrary, the combustion engine provides power to the hydraulic pump or pumps of the working hydraulic system in a more or less direct manner, e.g. by means of a mechanical gear wheel transmission connected between the output shaft of the engine and the input shaft of the pump or pumps. In other words, a rapid increase of the load on the working hydraulic system is transmitted to the combustion engine without any significant attenuation. Naturally, this may cause the combustion engine to stall or cause the power from the combustion engine to be fully consumed by the hydraulic system leaving the transmission line without any significant power. This may provide the operator of the wheel loader with the highly undesired impression that the engine has become too weak to move the wheel loader in an operable manner.
One way of solving the problem of meeting a sudden rapid increase of the load on the working hydraulic system is to run the combustion engine of the wheel loader at the higher end of its speed range. This provides a power margin which makes it easier for the combustion engine to meet a rapid load increase on the hydraulic system, e.g. time to recover by increasing the throttle. However, in general a higher rotational speed leads to significantly increased losses and thus increased fuel consumption. Therefore, with regards to fuel consumption it is better to run the combustion engine at lower rotational speeds. However, this will give a significantly reduced margin for the combustion engine to recover from sudden rapid increases of the load on the working hydraulic system.
In addition, to ensure that the hydraulic functions are equally fast at the lower rotational speeds, i.e. to ensure the same hydraulic flow at the lower rotational speeds, it is necessary to use larger pumps with higher displacement. A larger pump displacement requires a larger torque from the source driving the pump, i.e. from the combustion engine. In other words, if we move from higher rotational speeds towards lower rotational speeds for reducing losses and fuel consumption we will need hydraulic pumps with a higher displacement, which in turn leads to a higher torque load on the combustion engine. A higher torque load on the combustion engine at a lower rotational speed implies that the engine is utilized even harder. Hence, compared to the utilization at higher rotational speed for powering hydraulic pumps with a lower displacement it has now become even more difficult for the combustion engine to recover from a rapid increase of the load on the working hydraulic system.
Therefore, when designing a modern combustion engine for a working machine such as a wheel loader it is desirable to obtain high output torques at low rotational speeds and to obtain quick reactions on sudden rapid increase of the load on the working hydraulic system. To this end it is common to employ various turbochargers or air compressors. However, these and other solutions for reinforcing the performance of a combustion engine are commonly in conflict with increasingly harder emission regulations, particularly with respect to exhaustion gases and visible smoke emanating from engine responses to sudden rapid increases of the load on the working hydraulic system.
If the performance of the combustion engine cannot be boosted we can always meet a sudden rapid increase of the load on the working hydraulic system by at least temporary reducing the load on the hydraulic system. This can be achieved by limiting the displacement for the hydraulic pumps involved for a short initial period. In this way the torque required from the combustion engine can be temporarily reduced giving the combustion engine time to recover, e.g. time to recover by increase the throttle in a controlled manner. However, limiting the displacement for the hydraulic pumps will result in a reduced hydraulic flow which provides the operator of a wheel loader with an unwanted reduction of the performance of the hydraulic system, e.g. a reduced lifting speed and/or delayed lifting when lifting the bucket.
Considering the above there is clearly a need for a working machine provided with an improved ability to meet a sudden rapid increase of the load on the working hydraulic system with no or a reduced need for reinforcing the combustion engine or reducing the performance of the working hydraulic system.
It is desirable to provide a method of the kind referred to in the introduction, which creates conditions for operating a working machine in a more effective way.
According to an aspect of the present invention, a method is provided for operating a working machine provided with: a power source and a plurality of driving wheels; a working hydraulic system comprising at least one hydraulic pump powered by the power source for moving an implement on the working machine and/or for steering the working machine; a transmission line arranged between the power source and the driving wheels for transmitting torque from the power source to the driving wheels.
The method is characterized by the steps of:                detecting at least one operational parameter indicative of a load exerted on the working hydraulic system,        determining if torque needs to be added, in addition to a torque delivered by the power source, on the basis of a magnitude of the detected operational parameter in order to reduce the a load on the power source,        adding torque in case additional torque is determined to be needed.        
Adding torque by means of the above method is clearly providing a working machine with an improved ability to meet a sudden rapid increase of the load on the working hydraulic system with no or at least a reduced need for reinforcing the power source or reducing the performance of the working hydraulic system.
This is particularly so if the torque is added upon the detection of a transient increase of the load on the working machine, since a transient increase poses a particular risk of overloading or stalling the power source.
It is preferred that the torque is added to the transmission line. This is due to the fact that the working hydraulic system of a working machine is typically receiving power from the transmission line, which in turn is powered by the power source of the working machine.
It is also preferred that the torque is added by means of at least one electric machine, since this enables a flexible and compact design. An electric machine can also be powered by means of a plurality of power sources (e.g. batteries, generators, fuel cells etc), which provides an increased freedom in the design. Moreover, electric machines react fast on commands to provide a large torque already at low rotational speeds, which is beneficial considering that a rather large torque may have to be supplied fairly fast.
In addition it is preferred that at least one electric machine is arranged upstream a transmission unit that is typically arranged in ordinary transmission lines, or upstream a gearbox that is likewise typically arranged in ordinary transmission lines. In this way the electric machine does not have to work in both clockwise and counter clockwise directions to accommodate for both a forward and a reverse driving direction selected by means of the gear box. Moreover, arranging the electric machine upstream the transmission unit enables a more direct torque support to the working hydraulic system, since the hydraulic system is typically receiving power from a point in the transmission line that is located upstream the transmission unit.
It is furthermore preferred that the load on the working hydraulic system is predicted by monitoring at least one input command to the working hydraulic system. The commands are typically issued before the hydraulic system responds to the commands. Monitoring the commands will therefore provide additional time to add torque if needed, i.e. additional time before the hydraulic system will actually require more power from the power source
It is preferred that the invention is implemented in a working machine and in particularly in a wheel loader, since a typical wheel loader is operated with loads on the transmission line and the working hydraulic system that affect the power source substantially simultaneously creating transient loads of extraordinary magnitude.
It is also desirable to provide a working machine of the kind referred to in the introduction, which creates conditions for a more effective operation.
According to another aspect of the present invention, a working machine is provided with: a power source and a plurality of driving wheels; a working hydraulic system comprising at least one hydraulic pump powered by the power source for moving an implement on the working machine and/or for steering the working machine; a transmission line arranged between the power source and the driving wheels for transmitting torque from the power source to the driving wheels.
In addition the working machine comprises:                at least one detecting unit for detecting at least one operational parameter indicative of a load exerted on the working hydraulic system,        at least one control unit for determining if torque needs to be added, in addition to a torque delivered by the power source, on the basis of a magnitude of the detected operational parameter in order to reduce the a load on the power source,        at least one torque providing unit controlled by said control unit for adding torque in case additional torque is determined to be needed.        
The working machine displays the same or similar advantages as the method described above.
Further advantages and advantageous features of the invention are disclosed in the
following description.